Serpentine Belt Inspection and Replacement on Smart Fortwo 451 / 453
Service interval: Inspect at every oil change · Replace at signs of wear or roughly every 60,000 mi
Tools you'll need
- 15mm wrench or 1/2-inch breaker bar to relieve the auto tensioner
- Flashlight
- Clean rags
- Phone camera (photograph the routing before you pull the old belt)
What this is and why it matters
The serpentine belt is the single rubber belt that drives the alternator, the water pump, and on cars with factory air the AC compressor. If it snaps you lose charging, you lose cooling, and the car will overheat in minutes. The good news is this belt almost always tells you it's tired before it lets go — cracks across the ribs, glazing on the back, fraying at the edges. A two-minute look at every oil change keeps you ahead of it.
The Smart Fortwo 451 (M132 naturally aspirated and Brabus turbo) and the Smart Fortwo 453 both use a single serpentine belt with an automatic spring-loaded tensioner. The routing diagram is printed on the underside of the engine cover — lift the cover, flip it over, look at the sticker. That's the map.
The Smart Fortwo 450 does not use this layout. The 450 has separate, simpler accessory belts and a different alternator drive. If you're on a 450, this guide doesn't apply — see the 450 alternator soft-seize / no-crank guide for the 450-specific concerns.
What you'll need
A 15mm wrench or a 1/2-inch breaker bar fits the auto tensioner pulley bolt. You'll use that to swing the tensioner away from the belt. A flashlight helps because the engine bay is tight. Clean rags for any grime on the pulleys. And before you remove anything, pull out a phone and photograph the existing belt routing from a few angles. The diagram on the engine cover is right, but a real photo of your specific car is faster to read when you're trying to thread the new belt with one hand.
A new belt from Continental, Gates, or Dayco runs $25-50. Genuine Mercedes is more expensive and not usually worth it on a wear item like this. If the auto tensioner itself is chattering, leaking grease, or not springing back firmly when you release it, plan on replacing it at the same time — that's another $60-150 part.
Step by step
- Pop the engine cover at the rear of the car. Note where the routing diagram sticker is on the underside. You'll come back to it.
- Photograph the current routing. Multiple angles. Phone flash on. Belt path around alternator, water pump, AC compressor, idler, and tensioner.
- Inspect the old belt. Cracks running across the ribs (perpendicular to belt travel), glazing that makes the ribs look shiny instead of matte, fraying at the edges, or visible cord through the rubber all mean replace. If the belt is still soft, matte, and uncracked it's fine.
- Relieve the tensioner. Find the auto tensioner — the pulley with a spring-loaded arm. Put your wrench or breaker bar on its center bolt and rotate it away from the belt. The arm swings; the belt goes slack.
- Slip the old belt off. Hold the tensioner in the relieved position and walk the belt off the pulleys, usually starting at the alternator or AC pulley because they're the smoothest grooved.
- Route the new belt per your photo and the cover sticker. Get it onto every pulley except the last one (usually the tensioner pulley itself or the alternator).
- Release the tensioner slowly while holding the new belt aligned. The spring takes up the slack. Double-check every pulley has the belt seated in the grooves — not riding up on a flange.
- Spin the engine over by hand or start it for a few seconds and watch the belt track. It should run dead straight in the grooves with no chatter or squeal.
Common gotchas
The auto tensioner is spring-loaded and stronger than it looks. Keep your fingers clear of the pulley pinch points when you release it. A slip with the wrench can pinch a hand against the engine.
Belt seated on a flange instead of in the grooves is the most common mistake. It will look almost right and then either squeal immediately or shred itself in 50 miles. Walk every pulley with a finger before you start the engine.
Routing the belt mirror-image of the diagram happens to almost everyone the first time. The diagram view and your view are different. Trust the photo you took before disassembly more than the sticker.
If the belt squeals after install and you're sure the routing is right, the AC compressor clutch or the alternator pulley can drag and load the belt unevenly. That's a separate repair, not a belt problem.
When to skip DIY
Skip if the auto tensioner is seized or visibly damaged — that's a different procedure with the tensioner removal and you want a shop with a Smart-friendly lift to handle it. Skip if you don't have clear access from above without dropping the rear cradle (some 453 trims with factory AC are tighter). And skip if the belt is shredded and wrapped around accessories — that often means a pulley or bearing seized first, and the cause needs to be found before a new belt goes on or the new belt will be junk in a week.
A shop charges $60-150 in labor for this on top of the belt. If you're not set up for it, that's a reasonable spend.
Parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Serpentine belt (Continental, Gates, Dayco, or genuine Mercedes) | $25-50 | Search Google |
| Auto tensioner assembly (only if it's chattering or not holding tension) | $60-150 | Search Google |
Prices are rough community-reported ranges, not quotes. Aftermarket vs. genuine Mercedes parts swing the spread.
Manual references
- Browse Smart workshop manuals on smartcarmanuals.com — model-specific reference manuals on the home page; pick your chassis code section for torque specs and detailed procedures.
